The present invention relates to a circularly-symmetrical metal part having an outer surface that has a particular profile. It also relates to a polishing method enabling the particular profile to be obtained, and to apparatus for implementing the method.
There exist circularly-symmetrical metal parts which have outer surfaces that, in axial section, have profiles made up of non-uniform successions of peaks and valleys.
Such metal parts may, for example, be designed to be used as actuator rods.
In that type of use, in particular for hydraulic actuators, a major problem lies in preventing leaks at the hole through which the rod passes into the actuator body. Gaskets are thus disposed in said hole so as to rub against the rod, thereby sealing the actuator body. However, since the outer surface of the metal part forming the rod has a profile formed of a non-uniform succession of peaks and valleys, it has only a low bearing length ratio. As a result, the contact pressures between the gasket and the outer surface are distributed poorly, and this poor distribution of contact pressures gives rise to excessive wear on the gaskets, thereby leading to oil leaks. That is particularly troublesome since, to replace the sealing elements, it is necessary to stop machines whose high cost and intense operating cycle (they often operate non-stop round the clock) make stopping them for maintenance purposes extremely costly.
After the actuator has been in use for a certain length of time, the outer surface of the metal part forming the rod is xe2x80x9crun-inxe2x80x9d so that the bearing length ratio increases, thereby reducing the wear on the gaskets. It is thus important for the bearing length ratio to be as high as possible after a running-in time that is as short as possible. Currently, for running-in corresponding to a depth of cut (equivalent to a top portion of the profile of the outer surface being shaved off) equal to 40% of the total height of the profile (corresponding to the height of the highest peak measured from the lowest valley), the bearing length ratio obtained is about 20%. Unfortunately, for such a bearing length ratio, the wear generated remains very high.
In order to reduce the roughness of a surface, it is known, in particular from Document JP-A-61 061760, that a method of treating a surface exists that consists in blunting the peaks of the surface so as to limit the friction wear on a part moving relative to said surface. It has also been proposed, in particular in Document WO-A-95 02486, to use a method of treating a surface by hard anodization. However, the parts obtained by using those methods do not constitute satisfactory solutions to the above-mentioned problems.
In addition, the metal part is sometimes subjected to ambient conditions (humidity, heat, salt spray, etc.) giving rise to oxidation of said part. Such oxidation weakens the metal part and can even cause it to break. In order to avoid that, the outer surface of the metal part is conventionally covered in a stainless coating of chromium. Because of the non-uniformness of the profile of the outer surface, the layer of chromium inevitably has thin zones, and, in such zones, small cracks suffice to expose the outer surface to the surrounding environment. Such exposure then gives rise to harmful oxidation of the metal part.
An object of the invention is to provide a metal part whose outer surface has a particular profile making it possible to obtain a bearing length ratio that is high after a running-in time that is short.
To this end, the invention provides a circularly-symmetrical metal part having a longitudinal axis and an outer surface which, in axial section, has a general profile made up of a uniform succession of flattened convex arches.
Thus, the uniform succession of flattened convex arches procures a bearing length ratio that is high before running-in, so that said ratio reaches its maximum after a running-in time that is very short. The contact pressures between the members that move relative to one another are thus distributed more widely. When the metal part in question is used as an actuator rod, the wear on the sealing elements is thus limited, and the risks of overheating and of seizing are minimized. In addition, the valleys separating the flattened arches from one another are distributed uniformly along the outer surface. In particular in the case of hydraulic actuators, these valleys constitute reserves of oil which facilitate lubrication since they are distributed uniformly along the metal part. Furthermore, such a particular profile makes it possible to deposit a metal coating layer of uniform thickness. The resistance to corrosion is thus further improved.
Advantageously, the outer surface of the metal part is covered with a metal coating layer that has an outer surface which, in axial section, also has a general profile made up of a uniform succession of flattened convex arches.
The metal coating layer covering the outer surface of the metal part is of constant thickness and offers good protection for the metal part. By means of its profile, which is also in the form of flattened convex arches, the coating layer has a bearing length ratio that is high before running in, so that the above-mentioned advantages are reproduced for the coated part, with, in particular low wear on the sealing elements.
Preferably, the shape of the arches and the periodicity of their succession are such that, for depths of cut lying in the range 20% to 50%, the outer surface has a bearing length ratio lying in the range 25% to 95%.
Advantageously, then, the shape of the arches and the periodicity of their succession are such that, for a depth of cut substantially equal to 40%, the outer surface has a bearing length ratio lying in the range 40% to 95%.
Such bearing length ratios at such depths of cut, constitute an excellent compromise between limiting wear on the sealing elements and maintaining sufficient reserves of oil to ensure that lubrication takes place correctly. They also facilitate depositing a coating layer of constant thickness when the metal part is covered with such a layer, with the outer surface of the part then having an analogous particular profile in the form of a succession of flattened convex arches.
The invention also provides a method of polishing the outer surface of a circularly-symmetrical metal part that can have shape defects, the method comprising the steps of setting the metal part in motion relative to a tool and of applying the tool against a zone of the outer surface of the part so that it exerts a force thereon, the tool moving in a tracking motion so as to track the outer surface of the metal part in a radial direction that is essentially normal to said metal part in the zone against which the tool is applied, so that the force exerted by the tool on the part remains constant throughout the entire duration of polishing, regardless of the shape defects present on the surface, until a general profile is obtained that, in axial section, is in the form of a uniform succession of flattened convex arches.
This polishing method thus makes it possible to obtain metal parts having at least one of the above characteristics.
Advantageously, the metal part is supported vertically beneath the tool in a zone of the outer surface that is symmetrical to the zone against which the tool is applied about the longitudinal axis of the metal part.
Finally, the invention provides apparatus for implementing the polishing method having at least one of the above-mentioned characteristics, the apparatus including means for supporting the part and setting it in motion relative to a tool, the tool being mounted on a structure that is mounted to move in a direction substantially normal to the outer surface of the part so as to be applied by an associated force actuator against said surface by exerting a force thereagainst, and means for controlling the displacement of the moving structure as a function of the shape defects on the outer surface of the part.
In a particular embodiment, the tool is essentially constituted by an abrasive belt running over a contact wheel and over a return pulley, the wheel and the pulley forming the moving structure on which said tool is mounted. Preferably, the axle of the return pulley is then mounted to slide in a vertical plane by means of an associated actuator so as to constitute a tensioner pulley.
According to another particular characteristic, a shoe is disposed vertically beneath the tool to support the metal part in a direction substantially normal to the outer surface of said part. Advantageously, then, the position of the shoe can be adjusted by means of an associated actuator in a direction substantially normal to the outer surface of the part. The wear on the shoe can thus be compensated, and its position can be matched to various part diameters for the part to be polished.
Other characteristics and advantages of the invention appear on reading the following description of a non-limiting particular embodiment and implementation of the invention.
Reference is made to the accompanying drawings, in which:
FIG. 1 is a conventional graphical representation showing a surface profile of ordinary type;
FIG. 2 is a simplified fragmentary view in axial half section of a metal part of the invention, showing the particular profile in the form of a succession of flattened associated arches;
FIG. 3 is a view analogous to the FIG. 2 view, showing a metal part of the invention, as covered with a metal-coating layer whose outer surface also has the particular profile in the form of a succession of flattened convex arches; and
FIG. 4 is a diagrammatic fragmentary side view of apparatus for implementing the polishing method enabling the particular profile of the metal part of the invention to be obtained.
FIG. 5 is a top plan view showing a series of rollers 10, 11 uniformly spaced apart for supporting the metal part 1 along its length with the rollers oriented for defining a V-shaped guide in which the metal part 1 is supported. A shoe 21 is provided on the side of the metal part opposite the contact wheel 13 between two pair of rollers 10, 11 for forming a rigid backing element to react against the force applied by contact wheel 13.